1. Field of the Invention
The invention pertains to traffic alert collision avoidance systems and more particularly to such systems that cooperatively function with Air Traffic Control Radar Beacon Systems which provide intruder aircraft radar beacon surveillance.
2. Description of the Prior Art
A Traffic Alert and Collision Avoidance System (TCAS) installed on an aircraft cooperates with Air Traffic Control (ATC) by utilizing ATC Radar Beacon Systems (ATCRBS) transponders installed in most aircraft world wide and the new Mode S Transponders to provide locations of other aircraft in the immediate vicinity of the TCAS equipped aircraft. TCAS provides surveillance within the ATC System by transmitting interrogating signals to the ATCRBS equipped aircraft and measuring the relative range, altitude, and bearing from the responses of the ATCRBS transponder. The measured data and the rate of change of this data are utilized by TCAS to provide a prediction of aircraft penetration into a predetermined TCAS protection volume within a subsequent flight time of approximately thirty seconds. When an aircraft penetration is predicted within this time, an advisory is given to the pilot to climb to a new altitude, maintain its present altitude, or descends from his present altitude.
A problem with this system is that ATCRBS equipped aircraft respond to a non-unique interrogation. Thus, all ATCRBS transponders answer to the same interrogation when detected. Mode-S transponders respond to a unique address and may therefore be individually interrogated. To provide adequate surveillance of ATCRBS equipped aircraft a procedure has been developed that segments the air space in the manner that permits only a limited number ATCRBS equipped aircraft to respond at one time. This procedure, known as Whisper-Shout interrogates aircraft with increasingly stronger signals so that aircraft at short ranges from the interrogating aircraft reply before aircraft located at longer ranges. Copending U.S. patent application Ser. No. 430,747 filed Oct. 31, 1989 by Gegory T. Stayton and assigned to the assignee of this application, incorporated herein by reference, fully describes the Whisper-Shout procedure. ATCRBS transponders reply after two interrogation pulses, spaced twenty-one microseconds apart have been detected. These transponders are designed to suppress a response when two pulses two microseconds apart are detected. To reduce the number of aircraft replying to the TCAS/ATCRBS interrogation signals, a suppression pulse, having an amplitude lower than the interrogation pulse amplitudes, is transmitted two microseconds before the first interrogation pulse, as explained in the aforementioned copending patent application. When an interrogated aircraft is at a range at which the lower amplitude first pulse, the suppression pulse, is not detected, the transponder replies after the reception of the two interrogation pulses. Consequently, as the signal level of the interrogation is increased by the interrogating TCAS for detection by transponders at the longer ranges, the suppression pulse and the first interrogation pulse will be detected at the shorter ranges and the responses from the ATCRBS equipped aircraft at these shorter ranges will be suppressed.
Replies of all transponders are detected by the TCAS receiver, whether triggered by the TCAS interrogator associated with that receiver, other TCAS interrogators, or by ground station interrogators. Detected replies triggered by interrogators other than the one associated with the detecting receiver, commonly referred to as "fruit", if synchronous with the interrogation transmissions of the interrogator associated with the receiver may cause the system to establish false target tracks. This may occur if, in three successive interrogation intervals, replies are received that match within one-third of a nautical mile (4.12 microseconds). Synchronous detections of responses triggered by interrogators other than that associated with the receiver may most likely occur in five known situations;
a. A ground station triggers replies from an intruder transponder which are detected by the receiver associated with the interrogator of interest. The ground station interrogator is transmitting pulses at a K millisecond repetition interval, while the interrogator of interest is transmitting pulses at a L millisecond repetition interval. When multiple ground stations interrogate an area, triggered responses to these interrogations may give rise to fruit synchronous with pulse repetition rate of the interrogator of interest.
b. Interrogating ground stations generally employ techniques to suppress responses to signals radiated on a side lobe of the system antenna. Some military ground stations, however, do not have such side lobe suppression transmission. Consequently, a transponder in close proximity to these ground stations will detect and reply to the side lobe radiated interrogating signals. These replies may increase the number of possible synchronous fruit detections by as much as twenty-five times.
c. Modern transponders are equipped with circuitry to detect the sidelobe suppression signals radiated by an interrogating station and to suppress responses to these interrogations when suppression pulses are detected. A transponder having side lobe suppression circuitry not operating may have the same effect as interrogator transmissions without side lobe suppression pulses.
d. TCAS interrogations are transmitted as a sequence of five (5) pulses which include a suppression pulse, of amplitude less than the interrogation pulses, transmitted two microseconds before the first interrogation pulse which, after twenty-one microseconds, is followed by a second interrogation pulse. Transponders not detecting the suppression pulse will respond to the interrogation after detecting the two twenty-one microsecond separated interrogation pulses. A pulse from any other TCAS interrogator, whether airborne or ground based, may be detected with twenty-one microsecond spacings between it and a detected suppression pulse, the first interrogation pulse, or the second interrogation pulse from the interfering interrogator thereby causing a response from the transponder in synchronism with transmitted interrogations. These replies will be given 19 microseconds, twenty-one microseconds, or twenty-three microseconds early. If these replies persist, false tracks of 1.55 nmi, 1.71 nmi, and 1.87 nmi closer in range will be generated. The most likely source of an interfering pulse is a side lobe suppression pulse from a ground station, which is transmitted two microseconds after the transmission of the first interrogation pulse.
e. As explained in the aforementioned copending application, it is possible in certain angular sections about the interrogating aircraft, for the suppression pulse from the interrogator of interest to have an amplitude that exceeds the amplitudes of the two interrogation pulses. It is therefore possible to generate a response when an interfering pulse is received twenty-one microseconds after this suppression pulse. The detection of the suppression pulse from the interrogator of interest and the interfering pulse produces a synchronous fruit reply two microseconds or 0.16 nmi after a reply that would have been received from a transponder replying to an interrogation from the interrogator of interest.
Situations similiar to those described above may also occur due to interference from TCAS interrogations from sources than the ground station interrogators. All of the above situations will be more fully explained during the description of the preferred embodiments.